Protective relay with synchronized phasor measurement capability for use in electric power systems

ABSTRACT

The relay system obtains voltage and current values from a power line and uses a first sampling element to sample the voltage and current values at selected intervals of time. The resulting sampled signals are used for power system-wide protection, control, monitoring and metering. The sampled signals are then resampled at a rate which is a selected multiple of the power system frequency. The results of the resampling are used by processing circuitry for protection functions including fault determinations.

TECHNICAL FIELD

[0001] This invention relates generally to the monitoring and protectionof electric power systems, and more specifically, concerns a powersystem relay apparatus which is capable of monitoring system-wideperformance, such as with voltage and current oscillography and harmonicanalysis and voltage/current synchronized phasor measurements, whilealso providing protection functions, such as line distance protectionfor fault determinations.

BACKGROUND OF THE INVENTION

[0002] Heretofore, system-wide power monitoring functions, which includesystem control as well as disturbance analysis and harmonic frequencyanalysis, for example, require sampling of the data from the power linereferenced to time, either a local time clock or an absolute timereference, such as from a global positioning system. Digital faultrecorders, for instance, require data (current and voltage values) fromthe power line at fixed time intervals to carry out voltage and currentoscillography and harmonic analysis. Typical sampling rates are 1000 (1k) samples per second or faster, with sampling being synchronized, asindicated above, to an internal clock or an external time source.

[0003] On the other hand, many power system protection functions, suchas line distance protection algorithms, require sampling at multiples ofthe power system operating frequency (typically 60 Hz), to avoid phasormeasurement errors. Phasors used by the protection algorithms aredeveloped from the voltage and current values but can containsignificant errors where the system frequency is other than nominal.Protective relays determine the power system operating frequency and usethat frequency information to produce a sampling frequency which is aselected multiple of the operating frequency. This arrangement reducespossible errors in phasor calculations to a minimum; however, theresulting phasor measurements are not referenced to absolute time, sothat synchronized phasor measurement applications to an entire powersystem are not possible.

[0004] As an alternative to the above-described systems, the powersystem operating frequency, once obtained, can be used to modify thecoefficients of digital bandpass filters which are used to filter thesampled input data. Such a system provides information suitable for someprotection functions, but also, since the original input data is sampledvia a time-based clock (either internal or external) to provide anoscillography and harmonic analysis capability, it does not have acommon i.e. absolute, time reference for the multiple protective relaysand other protective devices located at different points in the powersystem. Accordingly, such a system is not suitable for those protectiveapplications which require synchronized phasor measurements.

[0005] It is thus desirable to have a single, comprehensive systemcapable of producing synchronized phasor values, including such valuescapable of being used for system-wide control and disturbance analysis,as well as line protection functions within a single power system relaydevice.

SUMMARY OF THE INVENTION

[0006] Accordingly, the present invention is a protective relay forelectric power systems for system-wide control and analysis and forprotection, comprising: acquisition circuits for obtaining at least oneof the following: (1) voltage values and (2) current values from a powerline; a first sampling circuit for sampling said voltage and/or currentvalues at selected intervals of time; a first calculation system usingthe resulting sampled values to perform selected power system-widecontrol and analysis determinations; a frequency estimating circuit fordetermining the power system frequency; a second sampling circuit forresampling the sampled voltage and/or current values at a rate which isrelated to the power system frequency; and a second calculation systemusing the resampled voltage and current values to perform selectedprotection functions for the portion of the power line associated withthe protection relay.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram showing a prior art time-based samplingapparatus for electric power system recorders, such as digital faultrecorders.

[0008]FIG. 1A is a diagram similar to that of FIG. 1, using a globalpositioning receiver to provide an absolute time reference.

[0009]FIG. 2 is a block diagram of a prior art system using sampling atmultiples of the power system operating frequency for electric powersystem relays.

[0010]FIG. 3 is a block diagram of the system of the present invention.

[0011]FIG. 4 is an alternative system to the system of FIG. 3.

[0012] FIGS. 5-7 are diagrams showing various communication arrangementsbetween a relay device and other units, including a host computer and/oranother relay device, involving solicited and unsolicited messages.

BEST MODE FOR CARRYING OUT THE INVENTION

[0013] As discussed above, certain power system monitoring devices suchas digital fault recorders, acquire analog voltage and current valuesfrom the power line at fixed time intervals in order to carry outvoltage and current oscillography analysis as well as harmonic analysison the entire power system. Such a system is shown generally at 10 inFIG. 1, with a source of voltage V_(A) shown at 12. Typically, theanalog data will include all three voltage phases and all three currentphases from the power line.

[0014] The analog signal V_(A) (at a suitable magnitude level, providedby a voltage transformer) is directed to a low pass filter 14 and thento an A-D converter 16. This is conventional. A local clock source 18 inFIG. 1 operating at a specific selected sampling interval (block 20)samples the A-D converter 16 at a selected rate, e.g. 8000 samples persecond. The resulting time-sampled signal is sent to a conventionalprocessing system 24 for oscillography and harmonic analysis. Since thetime-sampled data preserves the frequency information of the powersystem, analysis of the power system frequency excursions can beperformed during power system disturbances.

[0015] The system of FIG. 1A provides synchronized phasor values (inaddition to oscillography and harmonic analysis) by processor 25 when anabsolute time reference is used, instead of a local clock, such as froma GPS (global positioning system) reference 26 in communication with atime synchronization system. The use of these synchronized phasormeasurements or values, also referred to as synchrophasor measurements,will be discussed in more detail below.

[0016] Sampling the input data at constant time intervals is typicallynot used for protection functions, such as line distance protection,because it can introduce errors in the protection functions. Also, asbriefly discussed above, traditional line distance protection requiressampling at some multiple of the power system operating frequency.Referring to FIG. 2, data from the power line 30, such as phase Avoltage (V_(A)) from a voltage transformer, is applied to a low passfilter 32. A system frequency estimation circuit 34 obtains the systemfrequency from the output of the low pass filter to develop a powersystem operating frequency (f_(SYS)) This frequency information ismultiplied by a factor k at 36 to obtain the system sampling frequencyf_(s), which is applied to the A/D converter 37. Many types of frequencyestimation circuits can be used in the present system. One example isdiscussed in the following reference: A New Measurement Technique forTracking Voltage Phasors, Local System Frequency and Rate of Change ofFrequency, Phadke et al, IEEE Transactions on Power Apparatus andSystems, Vol. PAS-102 No. 5, May 1983. The sampled data from the A-Dconverter 37 is passed through a digital bandpass filter 38. The outputfrom the bandpass filter is applied to conventional protection processorcircuitry 41.

[0017] The disadvantage of the system of FIG. 2 is that the phasormeasurements have no absolute time reference, which is necessary forsynchronized phasor measurement applications. The system thus does nothave the time synchronization necessary for system-wide monitoring,metering, protection and control functions.

[0018]FIG. 3 shows the system of the present invention, which producessynchronized phasor values (measurements), which are independent ofsystem frequency, and which hence can be used for certain system-wideprotection functions. System-wide protection, monitoring, metering andcontrol functions, including oscillography and harmonic analysis ofFIGS. 1 and 1A, can be accomplished from the time-based sampling fortime-based applications using synchrophasors. In addition, the inputsignals are resampled at multiples of the power system operatingfrequency to provide a typical range of protection functions, such asline distance protection and fault determinations.

[0019]FIG. 3 includes a local protection apparatus or device 42, alsoreferred to as an intelligent electronic device, such as a protectiverelay, and a remote apparatus 44. The remote apparatus is identical tothe local apparatus and the two communicate with each other over acommunications channel 46. The description below concerns the structureand operation of the local device; the same description is applicable tothe remote apparatus 44.

[0020] In more detail, data is acquired from the power line, typically,three phases of current (I_(A), I_(B) and I_(C)) and three phases ofvoltage (V_(A), V_(B) and V_(C)). Only V_(A) is shown in FIG. 3 forpurposes of simplicity of explanation. Element 50 includes conventionaltransformer devices (voltage transformers/current transformers) whichreduce the current and/or voltage values to a level appropriate for usein a microprocessor-based protective relay 42. A typical voltage outputof element 50 will be approximately 1.34 volts under normal operatingconditions.

[0021] The output from element 50 is applied to a low pass filter 52,the output of which is applied to an A-D converter 54. The acquired datais sampled at fixed timed intervals. The sampling signal is referencedto an external clock signal, such as from a GPS receiver 56, which is anabsolute time reference. The output from the GPS receiver 56 is appliedto a time synchronization element 58. To produce synchronized phasormeasurements, such an absolute time reference is required. The resultingsampled output from the A-D converter is applied to a calibrationcircuit 62, which accounts for any data acquisition errors which mayoccur in the data acquisition hardware, so that the data is alignedbetween devices 42 and 44. The output of the calibration circuit 62,which is typically provided at a relatively high sampling rate, forexample, 8000 samples per second, is applied to a processing circuit 64for conventional oscillography and harmonic analysis applications.

[0022] In addition, however, synchronized phasor values are determinedfrom the power line input voltage and current values. The synchronizedphasor output of circuit 64 is independent of system frequency, and canbe used in certain protection functions carried out by the localapparatus 42.

[0023] The determination of synchronous phasor values per se is known,as set forth in a paper entitled “Measurement of Voltage Phase for theFrench Future Defense Team Plan Against Loss of Synchronism” at IEEETRANSACTIONS ON POWER DELIVERY, Vol. 7, No. 1, January 1992. Thealgorithm in processing circuit 64 uses the input values V_(A), V_(B),V_(C), and I_(A), I_(B), I_(C), at 8000 samples per second, with anabsolute time reference. The processor 64 decimates, i.e. decreases, thenumber of samples, dividing by eight, to create voltage and currentsignals at 1000 (1 k) samples per second. Next, each input signal ismultiplied by the reference signals cos(2πt+β) and sin(2πt+β) where timet is the absolute time reference and β is a calibration adjustment forthe particular hardware used. In the next step, the multiplied signalsare demodulated with a low-pass filter to obtain the real and imaginaryparts of the V_(A), V_(B), V_(C) and I_(A), I_(B), I_(C) phasors. Therelay calculates these particular phasors every 50 milliseconds.

[0024] Next, the processor uses the angle information from the V_(A)phasor calculation in the above step and the magnitude calculations fromthe filtered quantity V_(AF) (the filtered fundamental A-pphase voltagequantity) to produce the A-phase voltage synchrophasor (V_(Async)). Therelay performs similar calculations for the other phasors. Eachresulting synchrophasor is associated with a particular time mark,referred to as time-sync. This time mark is referenced to absolute time.

[0025] Then the positive sequence quantities (e.g. V_(1sync)) are thencomputed from the three-phase current and voltage synchrophasors asfollows:

V _(1sync)=(⅓)(V _(Async) +aV _(Bsync) +a ² V _(Csync))

I _(1sync)=(⅓)(I _(Async) +aI _(Bsync) +a ² I _(Csync))

[0026] Where a=1 at an angle 120°. An alternative to the positivesequence voltage, referred to as Alpha Clarke components, for protectioncalculations is as follows:

V _(alphasync)=(⅔)(V _(Async) −V _(Bsync)/2−V _(Csync)/2); and

I _(alphasync)=(⅔)(I _(Async) −I _(Bsync)/2−I _(Csync)/2)

[0027] The protection functions which use the synchronized phasormeasurements, such as the synchronized positive sequence phasor values,include current differential protection, in which current values from alocal device, such as device 42, are used with current values from aremote device, such as device 44, to provide the protection function.The absolute time reference for the synchrophasors provides the abilityto carry out such protection functions and make fault determinations.Although synchronized phasor measurements are known per se, as indicatedby the above reference, the application of synchronized phasors in aprotective relay for line distance protection is not heretofore known.Hence, system-wide analysis capability as well as some protectionfunctions are available from the processing circuit 64 alone.

[0028] Referring again to FIG. 3, the output of the calibration circuit62 is also applied to a digital low pass filter 68, the output of whichis applied to a “down” sampling device 70, which decimates, i.e. dividesthe number of samples, such as down to 1000 samples per second from 8000samples per second. The down sampled data is applied to the remotedevice 44 on transmit line 71 over a communications channel 46 at a ratewhich is suitable for the bandwidth of the communications channel. Thedown sampled data output is also applied to a conventional frequencyestimator 72, which can be any of several known devices, as discussedabove. Frequency estimator 72 will provide an estimate as to the actualfrequency of the power line signal.

[0029] The output of frequency estimator circuit 72 is applied to oneinput 73 of a two position switch 74. The other input 75 is thetransmitted data from the remote device, on receive line 76 fromcommunications channel 46. The inputs to the switch 74 thus are localand remote power signal frequency information. The output of switch 74is applied as the sampling signal f_(sys) to a resampling circuit 78.There are two data inputs to resampler 78, one on line 79 from theoutput of the digital low pass filter 68 of local device 42 (the localsignal) and the other on line 81 from the remote device via the remotechannel 82 (the remote signal).

[0030] Both of these input signals are resampled at a frequency which isa selected multiple of the operating system frequency, e.g. 32·f_(sys)in the embodiment shown. Other multiples could be used. The local andremote resampled data appear on output lines 80 and 83, respectively.These signals are then applied through digital bandpass filters 84 and86. A switch 88 controls the application of either the local or remotefrequency sampled data to a conventional protection, metering andprogrammable logic circuit 90. The selection between the two dependsupon the requirements of the programmable logic relative to theparticular protection function.

[0031] The local resampled data is also processed at 94 to produce anRMS (root-mean-square) value. This RMS data is used for metering andprotection applications for the relay. The output of the circuit 90 isapplied to the remote device over communications channel 46, whilesimilar information from the remote device is applied to the protectioncircuit 90 to implement protection functions and to produce faultindications and to trip a circuit breaker, when appropriate, for thatportion of the power system covered by the relay.

[0032] The system of FIG. 3 in a single device provides an oscillographyand harmonic analysis capability because of the time-based sampling aswell as protection and metering functions using samples based onmultiples of system frequencies. The apparatus thus has a comprehensiveprotection, monitoring, metering and control capability. The use of anabsolute time reference, such as a GPS receiver, permits the generationof synchronized phasor (synchrophasor) values, which are used in theprotection functions. The protection capability includes communicationand coordination with remote devices.

[0033] A variation of FIG. 3 is shown in FIG. 4, in which the samplingof the input voltage and current data from the power line is based on alocal clock signal from source 99, providing a sampling frequency f_(s)for A-D converter 101, while a resampling circuit 100 uses an absolutetime reference from a GPS receiver 102 to provide synchronized phasorvalues. The system frequency is also estimated by circuit 104 andapplied to the resampler 100. The absolute time reference and the systemoperating frequency resample the data from A-D converter 101 at aspecific frequencies, depending on the application. One frequency can beused for protection and another frequency can be used for synchrophasormeasurement.

[0034] Synchronized phasor measurement data from the device 42 of FIG. 3can be reported in two different ways, unsolicited binary messages atspecific time intervals and solicited ASCII messages at specific times.One example is shown in FIG. 5, where two devices (intelligentelectronic devices, such as protective relays) 112 and 114 communicatewith a host computer 116 over conventional communication channels 118and 120, using a conventional CRC (cyclical redundancy check) errordetection method.

[0035] Unsolicited binary messages from the IED's to the host computer116 will typically contain the following data: the IED address that isused by the host computer to determine the data source; the samplenumber of the data; the data acquisition time stamp with the absolutetime reference; the power system estimated frequency; the phase andpositive sequence voltages and currents from the power line; anindication of correct time synchronization; a confirmation that the datapacket is ok; followed by general purpose bits; and lastly, an errordetection code. The host computer 116 will parse the received data fromseveral different devices communicating with it in the network,according to the time stamp and the sample number in each data packet.

[0036] With solicited messages, the devices respond to a command fromthe host computer 116 relative to a phasor measurement by reportingsynchronized phasor measurements of meter data (magnitude and angle forthe three phase currents and voltages) in the power system at specifictimes. This arrangement is used to take “snapshots” of thesynchrophasors at specific times across the entire power system, which,when put together, show the power system condition at all points in thesystem at one specific point in time. A sample report is shown in FIG.6.

[0037] With relay-to-relay unsolicited binary messages (FIG. 7), a localrelay 130 using unsolicited binary messages can use a remote relay 132as a reference, with a communications channel 134. The data packettransmitted is similar to the unsolicited message shown in FIG. 6. Thedata packet contains one or more voltage and/or current values. Normallythe positive sequence voltage is transmitted. The local device (relay)130 uses the time stamp information to align the local and remote datapackets. For example, the local angle information ANG_(LOCAL) can bealigned with the remote angle ANG_(REMOTE) to obtain the angledifference between the two ends of the line. This angle difference canthen be used by the relay logic to perform control or protectionfunctions, with fixed or programmable logic.

[0038] For those messages, such as the unsolicited binary messagesdescribed above, which use positive sequence voltage values determinedfrom the synchronized phasor measurements as a reference, an alternativeknown as the Alpha Clarke voltage can be used. The Alpha Clarke voltageis a better reference choice than the positive sequence voltage becauseit is easier to calculate and hence reduces the relay-processing burden.The Alpha Clarke components are calculated as described above.

[0039] Hence, a system has been described which is capable of producingelectric power system control and analysis information as well asprotection functions for the system. In one arrangement, the absolutetime reference is used to produce a synchronized phasor measurement ofvoltages and/or currents for current or voltage differential analysis,while in another arrangement, the frequency is combined with frequencyestimation techniques and with re-sampling in order to provideinformation which can be used for other types of protection such as linedistance protection.

[0040] Although a preferred embodiment of the invention has beendisclosed for purposes of illustration, it should be understood thatvarious changes, modifications and substitutions may be made in theembodiment without departing from the spirit of the work, which isdefined by the claims which follow.

What is claimed is:
 1. A protective relay for electric power systems forsystem-wide control and analysis and for protection, comprising:acquisition circuits for obtaining at least one of the following: (1)voltage values and (2) current values from a power line; a firstsampling circuit for sampling said voltage and/or current values atselected intervals of time; a first calculation system using theresulting samples to perform selected power system-wide control andanalysis determinations; a frequency estimating circuit for determiningthe power system frequency; a second sampling circuit for resampling thesampled voltage and/or current values at a rate which is related to thepower system frequency; and a second calculation system using theresampled voltage and current values to perform selected protectionfunctions for the portion of the power line associated with theprotective relay.
 2. An apparatus of claim 1, wherein one selectedfunction is line distance protection.
 3. An apparatus of claim 1,wherein the first sampling circuit has an absolute time reference.
 4. Anapparatus of claim 3, wherein the absolute time reference is provided bya global positioning system.
 5. An apparatus of claim 1, including acalibration circuit for modifying the resulting sampled signals toaccount for errors in acquisition of data.
 6. An apparatus of claim 1,including a down sampling circuit for decreasing the number of samplesprovided by the first sampling element.
 7. An apparatus of claim 1,wherein the resampling circuit is responsive to data from the protectiverelay and also from another relay which is remote from said protectiverelay on the same power line.
 8. An apparatus of claim 7, wherein theestimated power system frequency is selected from the local protectiverelay or the remote protective relay.
 9. An apparatus of claim 7,wherein the resampled signals from both the local protective relay andthe remote protective relay are applied to bandpass filters which havefixed coefficients that do not depend on the power system operatingfrequency.
 10. An apparatus of claim 7, including a calculation circuitfor determining the RMS (root means square) value of the resampledsignals from the local protective relay, wherein the RMS values are usedfor metering functions.
 11. A protective relay for electric powersystems using synchronized phasor measurements for system-wide controland analysis and for power line protection, comprising: voltage andcurrent acquisition circuits for obtaining voltage and current valuesfrom a power line; a sampling circuit for sampling the voltage andcurrent values at selected intervals of time, wherein the sampling isbased on an absolute time value reference; a first calculation systemusing the sampled signals to perform selected power system-wideprotection, control and analysis determinations and for producingsynchronized voltage and current phasor values from the acquired voltageand current values, the synchronized voltage and current values beingsubstantially independent of system frequency for protection and controlfunctions; and a second calculation system responsive to synchronizedphasor values from the said protective relay and from another relaywhich is remote from said protective relay on the same power line. 12.An apparatus of claim 11, including communication means for transmittingunsolicited binary messages containing synchronized phasor values fromthe protective relay to a host computer, wherein the binary messagescontain the address of the protective relay, an absolute time stamp, asample number for the data being transmitted, the power system estimatedfrequency, and the synchronized phasor values for the voltages andcurrents on the power line for said sample with respect to absolutetime.
 13. An apparatus of claim 11, wherein the relay includes acommunication circuit responsive to a request from a host computer toreport the synchronized phasor values of voltage and currents present onthe line at specified times, wherein the synchronized phasor values froma plurality of protective relays in the power system are combined by thehost computer to provide an indication of the operating condition of thepower system at said specified times.
 14. An apparatus of claim 11,wherein the second calculation system uses the absolute time stampinformation from a remote relay on the power line to align data,including both magnitude and angle, from the local and remote sourcesand further uses the aligned magnitude and angle information to performthe protection and control functions.
 15. An apparatus of claim 10,wherein the synchronized phasor values are used to produce an AlphaClarke component voltage useful as a substitute for positive sequencevoltage in synchronized phasor protection applications.